In a semiconductor laser device having an edge-emitting type laser chip, a plurality of laser elements electrically isolated from one another by isolation grooves on a main surface of a semiconductor substrate are formed in an array. The semiconductor substrate is, for example, made of gallium arsenide. The semiconductor substrate is mounted on an laser chip mounting area of a main surface of a sub-mount so that the semiconductor substrate falls on its main surface. The sub-mount is, for example, made of silicon carbide. In the laser chip mounting area of the sub-mount, a plurality of electrodes are disposed oppositely to electrodes of the laser chip respectively. Solder layers are formed on the surfaces of the plurality of electrodes respectively. Through the solder layers, the electrodes of the laser elements are electrically and mechanically connected to the electrodes of the sub-mount respectively.
On another area of the sub-mount than the laser chip mounting area, a plurality of bonding pads for connections with bonding wires are formed in positions where the wires will not block laser beams when the wires are attached. The other ends of the bonding wires are connected respectively to one-end sides of a plurality of leads which are provided to penetrate a flange of a package and electrically isolated from the flange. Thus, a current can be applied from the outside of the package to each semiconductor laser element.
In order to suppress heating of the laser elements, the sub-mount is bonded to a heat sink which is made of a highly thermal conductive material such as copper. Further, the heat sink is mechanically bonded to the flange portion by brazing.
To improve the yield of the semiconductor laser device, however, there is a problem that the laser chip is stressed due to a difference in linear expansion coefficient between the sub-mount and the heat sink. The difference may appear when the sub-mount mounted with the laser chip is bonded to the heat sink.
Assume that gallium arsenide is used as the primary component of the laser chip, silicon carbide or aluminum nitride is used for the sub-mount, and copper, copper-tungsten or the like is used for the heat sink. In this case, as is apparent from Table 1, there is a large difference in linear expansion coefficient between the sub-mount and the heat sink.
The sub-mount and the heat sink are generally bonded by soldering with a die bonder. That is, a small piece of foil-like solder is placed on the heat sink, and the sub-mount is then positioned suitably relatively to the heat sink so as to cover the small piece of solder. The sub-mount is pressed and fixed by a pressing jig. The whole of a work including the sub-mount and the heat sink is heated to a temperature not lower than the melting point of the solder by a heater. Thus, the small piece of solder is melted to bond the sub-mount and the heat sink.
During the heating, the small piece of solder between the heat sink and the sub-mount is melted into liquid. The sub-mount has not yet been bonded to the heat sink till then. Therefore, the heat sink and the sub-mount bonded with the laser chip expand individually. On the other hand, the laser chip and the sub-mount have been already bonded. Accordingly, the sub-mount is deformed in conformity with the expansion of the laser chip having a large linear expansion coefficient. However, the sub-mount is not a light emitting element. In addition, the difference in linear expansion coefficient between the laser chip and the sub-mount is small. Therefore, stress acting on the sub-mount at this time causes no problem.
Next, the heater is turned off, and nitrogen gas is blown against the work so as to cool the work. When the temperature of the work is below the melting point of the solder, the sub-mount and the heat sink are bonded by the solder. After that, the nitrogen gas is blown till the temperature of the work is below about 100° C. After the blowing of the nitrogen gas is terminated, the work is cooled down to the room temperature which is, for example, set at 25° C.
During the cooling, the heat sink and the sub-mount are bonded, and the sub-mount and the laser chip are bonded. As the temperature drops down, the sub-mount and the laser chip are deformed in conformity with the shrinkage of the heat sink having the largest linear expansion coefficient. Thus, stress occurs in proportion to the quantity of the deformation.
With reference to deformation at a high-temperature when the solder between the sub-mount and the heat sink is melted, the peripheral portion of the heat sink is deformed more largely than the central portion thereof with the decrease of the temperature as shown in FIGS. 4A and 4B.
The laser chip and the sub-mount are also deformed with the shrinkage of the heat sink. As for the laser chip and the sub-mount, therefore, their peripheral portions are deformed more largely then their central portions respectively.
Assume that there are three light emitting points 6 in one light emitting edge as shown in FIGS. 4A and 4B. In this case, it is understood that there is a difference in quantity of deformation between the light emitting point at the center and each of the light emitting points at the opposite ends. Stress acts on an active layer including a light emitting point in the laser chip in proportion to the quantity of deformation. Therefore, there occurs a difference in stress between the light emitting point at the center and each of the light emitting points at the opposite ends.
The stress acting on the active layer affects light output characteristics such as an oscillatory wavelength, a threshold current, etc. It is also known that the stress affects reliability such as a life length or the like as disclosed in Non-Patent Document 1. Therefore, there occurs a difference between the light emitting point at the center and each of the light emitting points at the opposite ends as to light output characteristics such as an oscillatory wavelength, a threshold current, etc. or reliability such as a life length or the like.
For example, when the semiconductor laser device is applied to a writing light source in a laser printer, there is a problem that the difference in oscillatory wavelength appears as a difference in image position or image spot diameter between one laser beam and another, and the difference in threshold current makes it difficult to control the light outputs of laser beams.
There is another problem. That is, with increase of a distance D between the light emitting points at the opposite ends, a difference in light output characteristics such as an oscillatory wavelength or a threshold current between the light emitting point at the center and each of the light emitting points at the opposite ends is enlarged. Thus, reliability such as a life length as a semiconductor laser device having three light emitting points deteriorates extremely.
Lead-tin solder which has been often used for bonding a sub-mount and a heat sink in the background art contains lead. Lead was designated as a toxic substance in the EU RoHS Directive which was effective in July 2006. It will be therefore difficult to use the lead-tin solder in the future. For this reason, silver-tin solder, gold-tin solder, etc. are often used as solder materials. The melting point of the lead-tin solder is about 180° C., while the melting point of the silver-tin solder is about 230° C. and the melting point of the gold-tin solder reaches 280° C. As the melting point of the solder increases, a change in temperature during heating and cooling is enlarged so that the shrinkage of the heat sink during the cooling also increases. Thus, stress acting on the laser chip also increases.
In a multi-channel laser chip, the distance D between the light emitting points at the opposite ends of the laser chip may increase further. Assume that L designates a length of a substance, α designates a linear expansion coefficient of the substance, and ΔT designates a change of temperature of the substance. A length change ΔL of the substance in the temperature change ΔT can be expressed by:ΔL=L×α×ΔTAs is understood from this, stress acting on the laser chip is enlarged in proportion to the distance between the light emitting points at the opposite ends in the laser chip.
Though not shown in FIGS. 4A and 4B, assume that there are two light emitting points in one light emitting edge. The two light emitting points are generally disposed symmetrically with respect to a central line which is not shown in FIGS. 4A and 4B. Due to the symmetric arrangement, there occurs no difference in quantity of deformation due to decrease of temperature between the two light emitting points. When there is one light emitting point in one light emitting edge, the light emitting point is generally disposed at the center of the laser chip. Therefore, the quantity of deformation due to decrease of temperature is slight.
As has been described above, it is understood that the problem of the action of stress on an active layer including a light emitting point and the difference in stress between one light emitting point and another due to decrease of temperature during cooling when a sub-mount and a heat sink are bonded by soldering is conspicuous when there are three or more light emitting points in one light emitting edge.
The problem described above has been already known. Solutions to the problem have been proposed.
Patent Document 1 (JP-A-2005-259851) discloses a method in which a groove is provided in a surface of a sub-mount in contact with a heat sink so as to reduce the contact area between the sub-mount and the heat sink and thereby reduce deformation in the sub-mount and a laser chip. However, the shape of the sub-mount is so complicated that it is more difficult to mold the sub-mount. Thus, there is a problem that the cost increases. Solder enters the groove in the lower portion of the sub-mount when the sub-mount and the heat sink are bonded. Thus, there is another problem that a larger quantity of solder is required in comparison with that in a sub-mount with no groove.
Patent Document 2 (JP-A-5-315705) discloses a method using a configuration where a difference between a laser chip and a sub-mount as to values obtained by multiplying their linear expansion coefficients by the resonator length of a laser element respectively is not larger than a predetermined value. When this is applied to the present invention, the resonator length of a laser element can be regarded as a distance between light emitting points at opposite ends in a laser chip. As described herein, however, only reduction of the difference in linear expansion coefficient between a laser chip and a sub-mount is not effective to reduce the deformation of a heat sink due to cooling when the sub-mount is bonded thereto.
A semiconductor laser device may have a configuration where a laser chip is bonded to a heat sink without putting a sub-mount therebetween. In this case, for example, assume that gallium arsenide is used for the laser chip, and copper-tungsten is used for the heat sink. By this configuration, the linear expansion coefficient ratio of the heat sink to the laser chip can be suppressed to be low. Stress acting on an active layer including a light emitting point due to decrease of temperature during cooling when the laser chip and the heat sink are bonded by soldering is slight, and a difference in stress between one light emitting point and another counts for nothing.
Non-Patent Document 1: Atsumi and other three, “High-Power Laser Diode for Automotive Laser Radar”, Denso Technical Review, Vol. 9, No. 2, 2004
Patent Document 1: JP-A-2005-259851
Patent Document 2: JP-A-5-315705